Method for measuring filter efficiency

ABSTRACT

An improved process of passing an aerosol mixture through a filter. The  asol mixture solely contains about 70 to 76% isostearic acid, about 6 to 7% isopalmatic acid, about 7 to 11% myristic acid, and about 4 to 5% palmitic acid.

FIELD OF USE

An improved method of testing a filter for gas masks, respirators, andpersonnel protective equipment.

BACKGROUND OF THE INVENTION

The present invention relates to a method for the generation of a nearlymonodispersed aerosol in filter-testing penetrometer machines.

The present invention is superior to the previous process in that itemploys a candidate mixture as a replacement for dioctyl phthalate(DOP), which is a suspected carcinogen. In the following discussion, theterm "Candidate Material" will be used to designate aerosol mixturecontaining about 70 to 76% isostearic acid, about 6 to 7% isopalmaticacid, about 7 to 11% myristic acid and about 4 to 5% palmitic acid. Thecandidate mixture is manufactured by Henkel Corporation, Emery Group,11501 Northlake Drive, P.O. Box 429557, Cincinnati, Ohio 45249. Thecandidate mixture has been identified by us as a thermally stablematerial of low toxicity.

For several decades, the U.S. Army has produced hot smokes using DOP asthe standard material in the performance of nondestructive gas mask andfilter serviceability testing. Hot smokes are aerosols produced using amethod of thermally-generated vaporization and recondensation (selfnucleation) of particles. Heated air passes across the surface of aheated liquid (DOP); cooler air then merges with the vapor causingrecondensation of an aerosol or "hot smoke." The U.S. Army SurgeonGeneral has designated DOP as a suspected carcinogen and has prohibitedor severely restricted its use in smoke-generating machines used to testU.S. Army masks, respirators, filters, and other personnel protectionequipment.

The ATI model TDA-100 (Q-127) monodispersed filter penetrometer ismanufactured by Air Techniques, Inc. (ATI), a Division of HamiltonAssociates, Inc., 1716 Whitehead Road, Baltimore, Md. 21207. Adescription of this equipment is given elsewhere in this disclosure.Thus, the candidate material and process have been shown to beacceptable to the manufacturer for specification in his new machines.

DESCRIPTION OF FIGURE

FIG. 1 is a diagram showing flow paths for aerosol generation,measurement, and filter test flows in the TDA-100 machines.

SYSTEM OVERVIEW

The TDA-100 machine produces hot smoke by passing a heated air streamacross a heated reservoir, collecting vapor from DOP or the heatedcandidate material. This vapor stream merges with a cooler quenching airstream which causes the vapor to condense into an aerosol of particlesin the submicron size range. The aerosol mean particle size, width ofdistribution, and mass concentration is controlled by the temperatureand volume ratios of the air streams and the temperature of the heatedDOP or candidate material. The recondensed aerosol passes into an agingchamber to allow uniform mixing of the aerosol. From the aging chamber,a sample of the aerosol is piped to an optical nephelometer called amechanical owl which indicates mean particle size by the intensity ratioof mutually perpendicular components of polarized scattered light viewedat an angle of 90 degrees from the incident beam. From the mechanicalowl analyzer, the aerosol is piped to a "LAS-X" laser aerosolspectrometer. The LAS-X is manufactured by Particle Measuring Systems,Inc., 1855 South 57th Court, Boulder, Colo. 80301. The LAS-X measuresparticle frequency in contiguous class intervals from which onedetermines the mean particle size, expressed as geometric mean diameter(GMD), and width of particle distribution, expressed as geometricstandard deviation (GSD). Aerosol from the aging chamber is also pipedto the filter test chuck for filter efficiency analysis. The chuck is apneumatically-operated device which holds and seals the filter canisterto be tested. Aerosol passing through the filter flows through a lightscattering chamber. A measurement of scattered light intensity upstreamand downstream of the filter indicates the percent of the originalaerosol which passes through the filter, i.e., the filter's efficiency.

SYSTEM OPERATION

Air for smoke production is generated by a blower contained within theinstrument (1, FIG. 1). The air generated by the blower is filteredbefore use by an in-line air filter (2, FIG. 1). Filtered air forproducing smoke is regulated by a pressure regulator (3, FIG. 1), set to6 psi. Immediately downstream of the pressure regulator, the air streamsplits, one portion is used for vapor pick-up air, the other for vaporquench air.

The vapor pick-up air is controlled by a value (7, FIG. 1). The flow isread from a flowmeter (8, FIG. 1). The vapor pick-up flow has anadjustment range of 0 to 30 liters per minute (LPM). Newer TDA-100models may have a broader adjustment range. After passing through theflowmeter, the vapor pick-up air is heated by the in-line vapor pick-upair heating element (9, FIG. 1). The heating element receives avoltage-adjusted to maintain a vapor air temperature of 165° C. Thevapor pick-up air stream passes over the heated candidate material inthe hot pot, where is picks up vaporized candidate material before itmerges with the quench air.

The quench air control value (4, FIG. 1) controls the air flow. Thequench air flow rate can be adjusted from 0-100 LPM. The flow rate isread on the quench air flowmeter (5, FIG. 1) An in-line heating element(6, FIG. 1) controls the quench air temperature. A variac controller isused to adjust the voltage to the quench air heating element and controlpartial size. The voltage range is variable from 0-110 volts.

The candidate material operating temperature is maintained in the "hotpot" (10, FIG. 1) by a liquid heating element (11, FIG. 1). Athermocouple (12, FIG. 1) automatically monitors the candidate materialtemperature. An Athena thermoregulator automatically controls thecandidate material temperature.

After the air and the quench air merge, the recondensed smoke mixes andstabilizes in the aging chamber (13, FIG. 1). From the aging chamber,the smoke can exit through three pipes:

1. The smoke is piped to the owl for particle sizing.

2. The smoke is piped to the chuck to be used in filter penetrationtesting.

3. The remaining smoke which is not used for sizing or testing isvented.

A portion of the smoke, from the aging chamber is drawn through the"mechanical owl" (14, FIG. 1) to be used in smoke particle sizing. The"mechanical owl" consists of a smoke chamber with photomultiplier tubesat each end to view the white light scattered at right angles to acollated beam passing through the chamber. Before reaching eachphotomultiplier tube, the light is passed through a fixed polaroid diskand a rotatable polaroid disk. The fixed disks at each end are arrangedwith their axes of polarization. The angle of rotation relative to oneof the fixed disks is measured on a vernier protractor scale. Acalibration procedure is available to match the photomultiplier tubegains. In use, the signal outputs are subtracted and the result read ona meter. For a particular angular setting of the paired rotatablepolaroids, a null meter reading indicates equal scattered lightintensity viewed through the two polaroid disks with mutuallyperpendicular axes of polarization. This angle has been uniquelycorrelated to particle size of monodisperse liquid smoke particles inthe submicron range.

In our research, a second method of particle sizing was used. Thismethod will not be available to operators in the field. This secondmethod of sizing gives a direct reading of the frequency of aerosolparticle sizes in a number of contiguous size intervals while themechanical owl reading above only indicates an average size. The methodof indicating particle size with the mechanical owl rather thanmeasuring a particle size histogram with a LAS-X has been used byoperators for decades. As long as the recommended machine settings areused, the size indication from the owl is acceptable. In the past,recommended machine settings and the owl measurements have been theoperator's only means of controlling aerosol GMD.

Candidate material smoke is drawn from the outlet of the mechanical owlanalyzed through a TSI model 3302 capillary diluter (16, FIG. 1). Thecapillary diluter dilutes the smoke from the mechanical owl by a factorof 2000:1. A portion of the diluted smoke is piped to the LAS-X. TheLAS-X laser aerosol spectrometer makes particle histogram measurementson the diluted smoke which are then analyzed and the results printedusing the Hewlett Packard Microcomputer (17, FIG. 1). This systemmeasures and prints smoke GMD and GSD. Smoke from the capillary diluterwhich is not used by the LAS-X is filtered by an in-line high efficiencyfilter (18, FIG. 1). The flow rate of sample smoke is controlled by thesample control valve (19, FIG. 1). The cleaned sample air is thenexhausted via the vacuum source. (25, FIG. 1).

Smoke for filter testing is drawn from the aging chamber through thechuck (22, FIG. 1). Smoke concentration is measured downstream of thetest chuck by a light scattering chamber (20, FIG. 1.) Filterpenetration is measured in percent. The 100% gain is set by closing thechuck and allowing the concentrated smoke to be sampled by thepenetrometer (20, FIG. 1). The penetration indicator is zeroed byopening the chuck and allowing clean air to be sampled by thepenetrometer. The penetration indicator (21, FIG. 1) indicates percentpenetration through the test filter. Smoke remaining in the test airstream after filter tests or penetrometer setup is filtered by a in-lineHEPA filter. Filter resistance is indicated by the filter resistancegauge. The test air flow is controlled by a valve (23, FIG. 1). Test airflow is indicated by test air flowmeter (24, FIG. 1). Test air isexhausted via a vacuum pump (25, FIG. 1) contained in the TDA-100.

RECOMMENDED MACHINE SETTINGS

The following machine parameters were found in our process to produce anaerosol, using our candidate material, which gave the same or bettersmoke test performance as those measured experimentally using DOP.Actual settings may differ slightly between machines

    ______________________________________                                        Candidate material temperature                                                                    155      degrees C.                                       "Owl" Setting       50       degrees                                          Particle size control                                                                             65-75    volts                                            Vapor Pick-up Flow  10       LPM                                              Quench Air Flow     90       LPM                                              ______________________________________                                    

A candidate material temperature of 155 C. was found to produce a massconcentration of approximately 85 mg/m³. To obtain higherconcentrations, the candidate material temperature was raised. An "Owl"setting of 35 degrees and the above vapor pick-up air/quench air ratiosgenerated GMDs of 0.2 micrometers (μm) with GSDs of approximately 1.23.These specifications met or exceeded those obtained using DOP, and werewithin the U.S. Army test requirements of 0.18-0.33 μm GMD and GSD of1.30.

PENETROMETER

The TDA-100 Monodispersed Aerosol Penetrometer incorporates the mostadvanced technology of unique design to make 0.3 micrometermonodispersed aerosol, measure and control the aerosol particle size andconcentration plus measure the percent penetration of the testedcomponent by the aerosol.

The TDA-100 is a basic apparatus consisting of three major components.They are:

1. The penetrometer itself consisting of the aerosol making andcontrolling equipment.

2. The particle size indicator and the mechanical analyzer which monitorthe aerosol particle size.

3. The percent penetration indicator and associated light scatteringchamber which measures the percent of aerosol penetrating the componentbeing tested.

There are many adaptations and possibilities for various chuck and testfixtures which enable testing of a great variety of samples ranging fromflat material to highly complex respirators.

In general, the TDA-100 operates as follows:

Compressed air, passing through a filter and moisture trap, is connectedto the penetrometer and regulated to a pressure of 6 pounds per squareinch gage (psig). The air is then divided into two streams, vapor anddiluent. The vapor stream flows at 20 liters per minute through apreheater, then into an aerosol generator and over the surface of liquidwhich is maintained at 165±2° C. The diluent stream is cooled by avortex tube and then heated by an electrical element. It bypasses theaerosol generator at a flow rate of 80 liters per minute and joins thevapor stream on the outlet side of the generator to make an aerosol. Theaerosol is passed into an aging chamber where it is stabilized. Duringtesting, aerosol flows from the aging chamber to the chuck or testfixture adaptation and through the component under test, such as afilter. As aerosol is continually being made when the penetrometer isoperating and testing is intermittent, the excess aerosol is exhaustedto the atmosphere from the aging chamber. The aerosol particle size ismaintained at a predetermined level by controls on the penetrometer andis monitored by the aerosol particle size indicator. This indicatorelectronically measures aerosol particle size from a sample of theaerosol continually passing through a mechanical analyzer. Thismechanical analyzer measures aerosol particle size by the degree ofpolarization of a light beam which passes through a sample of theaerosol. The particle size of the aerosol is controlled by adjusting thetemperature of the diluent air stream. A sample under test is subject toa concentration of aerosol of approximately 100 micrograms per liter.Using this concentration as a base line of 100%, the amount of aerosolpenetrating the sample under test is measured by the percent penetrationindicator. Such measurements are registered linearly on the meter. Thespecifications for the apparatus should be as follows:

1. Aerosol Generator: Produces 0.3 micron aerosol at a concentration of100 micrograms/liter.

2. Vapor Flowmeter: Ranges from 5-50 SLPM @35 PSIG.

3. Diluent Flowmeter: Ranges from 10-100 SLPM @35 PSIG.

4. Test Flowmeter: Ranges from 16-85 SLPM @5"HG.

5. Resistance Indicator: Optional.

6. Mechanical Analyzer: Measures light-angle refraction from 0-50 withfour Polaroid and three condensing lenses.

7. Particle Size Indicator: Solid state type, capable sensitivity of tendivisions to 1° rotation of Mechanical Analyzer, approximatesize--14"×8"×8".

8. Scattering Chamber: Forward light scattering, approximately 5"×5"×20"in size, with no dimming control and filter factor.

9. Percent Penetration Meter: Solid state type with ranges of 100%, 10%,1%, 1%, 01%. Approximate size--14"×8"×8". Three place digital read outoptional.

10. Vortex Tube: 5 cubic feet per min. capacity.

11. Mixing Chamber: Containing baffles with ports for exhaust, sample,inlet and test sample.

12. Vacuum Pump: Capable of delivering up to 85 SLPM L@5"HG pneumatic,silent operating type.

13. Air Operated Chuck: Manufactured to house customers' canisters ofvarying sizes, etc., to be tested.

14. Constant Voltage Regulator: 250 VA rating. Input of 95-130 VACoutput of 118 VAC 0.5%.

15. Control Panel: Consisting of master "ON-OFF" particle size control,solid state time proportioning liquid temperature control, chuck controlswitches.

AEROSOL MEASUREMENTS

The following information is provided (1) to clarify how aerosolparticle size distributions are represented, (2) to give U.S. Army smokeaerosol specifications for filter-testing penetrometer machines used totest respirators and mask canisters, and (3) to compare typicalperformance obtained using dioctyl phthalate (DOP) in our penetrometermachine with that obtained by us using our replacement material in ourpenetrometer machine using our process as described herein.

The U.S. Army requires these test smokes (aerosols) to meet thesespecifications:

(1) The geometric mean diameter (GMD), in micrometers (μm), of theaerosol must lie between 0.18 m and 0.33 μm. This is the count or numbermean of the distribution. That is, all particles in all size ranges arecounted, and a distribution is drawn showing the total number ofparticles in all ranges (a histogram). From this, a mean size isdetermined.

(2) The geometric standard deviation (GSD) of the distribution must notexceed 1.30. The GSD is a measure of the narrowness (width) or"monodispersity" of the particle size distribution. An aerosol ofparticles of all one size would have a GSD=1.00. This is impossible toachieve even with latex spheres that are used to calibrate theinstruments. The specified upper limit of GSD=1.30 insures that thewidth of the distribution is adequately narrow for desired tests. Bycomparison, aerosols produced by spraying (without vaporation andrecondensation) often have GSDs of 2.00 or more.

(3) The smoke concentration at the test chuck where filter canisters areinserted must be 100 mg/m³ plus or minus 20 mg/m³. Concentrations thatare too high can be reduced by process control adjustments. But good DOPreplacement smokes should produce at least 80 mg/m³ of smoke at thechuck.

In conclusion, an improved method of measuring the efficiency of aparticulate filter as described herein has been shown. This isaccomplished by passing an acidic aerosol mixture through a filter andmeasuring the geometric mean diameter to a specific degree by utilizingconventional equipment.

What is claimed is:
 1. In an improved process of passing an aerosolmixture through a filter and thereafter measuring the efficiency of saidfilter, the improvement consisting of said aerosol mixture solelyconsisting of about 70 to 76% isostearic acid, about 6 to 7% isopalmaticacid, about 7 to 11% myristic acid and about 4 to 5% palmitic acid. 2.The process of claim 1 wherein the concentration of said aerosol mixtureat said filter is 100 mg/m³ plus or minus 20 mg/m³.
 3. The process ofclaim 1 wherein said measuring of said filter efficiency is done bylight-scattering means.
 4. The process of claim 1 wherein the geometricmean diameter of said aerosol mixture is about 0.31 micrometers.
 5. Theprocess of claim 1 wherein the geometric standard deviation of saidaerosol mixture is about 1.25.